Here is a test case that demonstrates this:
The actual statistics show a "0" as the row count (ActRowsUsed) for the lowest EX_HASH_GRBY with id 2:
The reason is that the hash groupby reports its row count in the BMO stats. However, a partial hash groupby is not considered a BMO (Big Memory Operator), so no rowcount gets reported. The fix is to increment the rowcount in the generic stats entry that is present in both partial and full groupby operators.